Crystallization methods can be used to separate paraxylene (p-xylene) from a C.sub.8 aromatic starting material which contains ethylbenzene, as well as the three xylene isomers. Use is made of the fact that the melting point of the individual C.sub.8 isomers have significant temperature differences. P-xylene has a freezing point of 13.3.degree. C., metaxylene has a freezing point of -47.9.degree. C. and orthoxylene has a freezing point of -25.2.degree. C. However, conventional crystallization methods can be used to make p-xylene with a purity of over 99.5 wt. % only with great expense.
Crystallization processes to recover p-xylene from a mixture of C.sub.8 aromatics requires cooling the equilibrium feed mixture from reformate or xylene isomerization processes. Because it's melting point is much higher than that of the other C.sub.8 aromatics, p-xylene is readily separated in the crystallizer after refrigeration of the stream. In conventional p-xylene crystallization processes, the feed contains about 22 to about 23 wt. % p-xylene. In order to crystallize out most of the p-xylene from solution, the feed has to be cooled to as low as about -85.degree. to -95.degree. F. Conventional crystallization processes operate in the manner described in U.S. Pat. No. 3,662,013.
In conventional crystallization the maximum theoretical p-xylene recovery is fixed by the temperature of the coldest crystallizer in the crystallization unit. That temperature is limited by eutectic temperature, the temperature at which a second component, generally m-xylene, starts to crystallize and contaminates the p-xylene crystals. Given an equilibrium mixture of xylenes in the crystallizer feed, the coldest crystallizer is cooled to within 5.degree.-10.degree. F. of the eutectic temperature to maximize p-xylene recovery. Theoretically, the p-xylene recovery is limited to about 70% at the eutectic temperature. P-xylene recoveries of 60-65% are typical.
In a conventional two stage crystallizer. Equilibrium C.sub.8 aromatic feed is cooled to about -30.degree. to -40.degree. F. and mixed with second stage filtrate and then crystallized in a number of crystallizers in series, each crystallizer cooling the feed further, the coldest of which runs typically at about -80.degree. to -90.degree. F. The slurry solids and liquor, i.e. mother liquor, are separated by centrifuge. In the first stage, the solids become a wet cake with voids filled by the liquid containing only about 8-12 wt. % p-xylene. This low p-xylene liquid contaminates the crystals by 5-15%, depending on the drying efficiency of the centrifuge and prevents the p-xylene concentration from achieving the required 99.5+ wt. % purity. The remaining liquid is discharged as reject filtrate. This wet cake is either fully or partially melted and recrystallized or washed to remove the contaminants to achieve the required high p-xylene purity.
The second stage re-crystallizes the first stage product and filtrate p-xylene from the second stage recycle filtrate out of solution. The resulting slurry of crystals and mother liquor is centrifuged. The wet p-xylene crystals cake goes to the wash step, the remaining liquid is recycled filtrate. A controlled amount of the recycle filtrate is used to dilute the first stage product in order to control the slurry solids loading in the crystallizer. Typical centrifuges separate a slurry mixture containing no more than 35-45 wt. % solids. The second stage crystallizer operates at 0.degree. to 40.degree. F. and thus requires and processes a much smaller stream than the first stage and thus requires much less refrigeration.
The second stage cake voids are filled with liquid that is already rich in p-xylene, typically about 60-75 wt. %, and thus washing the crystals with product p-xylene can achieve a feed purity in the order of 99.5+ wt. % p-xylene.
A new approach to crystallization of p-xylene has now been found when processing a feed rich in p-xylene. It is an object of the present invention to provide an process for recovering p-xylene having a purity of at least 99.5 wt. % and preferably 99.8 wt. % from a feed rich in p-xylene.